13.5 Efficiency 389
The efficiency of motors is a function of load and speed. The electric motor manufacturers pro-
vide performance curves and tables for their products that show, among other information, the
efficiency of the motor.
You find pumps in hydraulic systems, the fuel system of your car, and systems that deliver
water to a city piping network. Pumps are also used in food processing and in petrochemical
plants. The function of a pump is to increase the pressure of a liquid entering the pump. The
pressure rise in the fluid is used to overcome pipe friction and losses in fittings and valves and
to transport the liquid to a higher elevation. Pumps themselves are driven by motors and
engines. The efficiency of a pump is defined by(13.19)
The efficiency of a pump, at a given operating speed, is a function of flow rate and the pressure
rise (head) of the pump. Manufacturers of pumps provide performance curves or tables that
show, among other performance data, the efficiency of the pump.Efficiency of Heating, Cooling, and Refrigeration Systems
Before we discuss the efficiency of heating, cooling, and refrigeration systems, let us briefly
explain the main components of these systems and how they operate. We begin with the cool-
ing and refrigeration systems because their designs and operations are similar. Most of today’s
air-conditioning and refrigeration systems are designed according to a vapor – compression cycle.
A schematic diagram of a simple vapor – compression cycle is shown in Figure 13.11. The main
components of the vapor – compression cycle include a condenser, an evaporator, a compressor,
and a throttling device, such as an expansion valve or a capillary tube, as shown in Figure 13.11.
Refrigerant is the fluid that transports thermal energy from the evaporator, where thermal
energy or heat is absorbed, to the condenser, where the thermal energy is ejected to the sur-
roundings. Referring to Figure 13.11, at state 1, the refrigerant exists as a mixture of liquid and
gas. As the refrigerant flows through the evaporator, its phase completely changes to vapor. The
phase change occurs because of the heat transfer from the surroundings to the evaporator and
consequently to the refrigerant. The refrigerant enters the evaporator tube in a liquid /vapor mix-
ture phase at very low temperature and pressure. The temperature of the air surrounding the
evaporator is higher than the temperature of the evaporator, and thus heat transfer occurs from
the surrounding air to the evaporator, changing the refrigerant’s phase into vapor.efficiency
power input to the fluid by the pump
power input to the pump by the motor
Jacuzzi
Pump
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